Midline referencing femoral sizing caliper

ABSTRACT

A sizing caliper for facilitating the selection of a femoral component of a knee prosthesis includes a caliper body, two drill guide bodies, a stylus tower configured to be slidably linked to each other wherein as the caliper body and the stylus tower are linearly displaced at an equal rate relative to the drill guide body portions and the drill guide holes remain located at the midpoint of an anterior/posterior dimension defined by the distance between the tip of the stylus and the base portion. The caliper body comprises a base portion configured to couple and decouple from the caliper body using a sliding mechanism.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/180,966, filed Jun. 13, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/179,003, filed Feb. 12, 2014, now U.S. Pat. No.9,364,346, which is a continuation of U.S. patent application Ser. No.13/540,782, filed Jul. 3, 2012, now U.S. Pat. No. 8,652,144, which is acontinuation of U.S. patent application Ser. No. 12/488,252, filed Jun.19, 2009, now U.S. Pat. No. 8,216,244, the disclosures of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present disclosure is related to sizing and locating a distalfemoral knee prosthesis for locating and securing the prosthesis inplace of the distal femur.

BACKGROUND

In order to size and position a femoral knee prosthesis, sizers orcalipers are used. The conventional calipers typically follow thereferencing method of how the prosthesis grows from size to size. Forexample, in some knee prosthesis systems, anterior referencing sizingscheme is utilized for the implants where a common anterior/posteriordistance from the tip of the anterior flange and the medial/lateralstabilizer pegs (perpendicular from the distal surface of the implant)is maintained for different sizes. The anterior referencing sizingscheme requires that the sizing caliper that sizes the distal femoralimplant must measure in the same manner, in which the device willmaintain a common distance for drill holes referenced in later steps inthe procedure and grow in the opposite direction (i.e. posteriordirection).

In other knee prosthesis systems, posterior referencing sizing scheme isutilized. In the posterior referencing sizing scheme, a commonanterior/posterior distance from the posterior condyles and themedial/lateral stabilizer pegs is maintained for different sizes.Similar to the anterior referencing systems, the posterior referencingsystem also imposes an asymmetric requirement on the sizing caliper. Thesizing caliper must measure in the manner in which the caliper maintainsa common distance for the drill holes referenced in later steps in theprocedure and grow in the opposite direction (i.e. anterior direction).

These conventional measuring instruments are dependent on the implantsystems that drive them, but create additional problems for the onesthey attempt to solve. For the case of an anterior referencing system,the attempt to prevent notching the anterior cortex (a condition wherean undercut is created in the anterior cortex resulting from improperposterior positioning of subsequent instruments) will create all boneresections to pull away from the posterior condyles. This can createissues with the flexion/extension gap and create a tighter implant thatlimits flexion or creating instability in the implant. Additionally,posterior condylar offset can be affected, resulting in conditions wherethe patient may not obtain as much flexion as desired by the surgeon.Conversely, for a posterior referencing implant, the attempt to maintainflexion/extension gap (and posterior condylar offset) kinematics couldcreate a condition where the surgeon may notch the anterior cortex, thuscreating a stress riser and possible fracture conditions. Additionally,mal-position or inaccurate sizing of a posterior referencing system maynegatively affect the patella-femoral gap, which may also impact theability for the replaced knee to achieve full flexion or extension.

SUMMARY

According to an embodiment of the present disclosure, a sizing caliperfor facilitating the selection of a femoral component of a kneeprosthesis to be attached to a resected distal end of a femur isdisclosed. The caliper comprises a caliper body configured for couplingto the resected distal end of the femur and be referenced to one or moreof the posterior femoral condyles of the femur, a drill guide body, astylus tower, and a graduated stylus slidably coupled to the stylustower. The graduated stylus is configured to be placed adjacent ananterior condyle surface of the femur, wherein the caliper body, thedrill guide body and the stylus tower are slidably linked to each otherby a linkage pin laterally extending there through in a manner therebywhen the caliper body and the stylus tower are linearly displaced inopposite directions relative to the drill guide body, the caliper bodyand the stylus tower are linearly displaced at an equal rate relative tothe drill guide body.

The sizing caliper enables locating the referencing holes on theresected distal end of the femur for subsequent chamfer resections ofthe femur to be referenced from the midline of the distal end of thefemur between the anterior cortex (anterior flange run-out of the distalfemoral implant) and the posterior condyles rather than strictly theanterior or posterior surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a femoral sizing caliper according to anembodiment of the present disclosure.

FIG. 2 is another perspective view of the sizing caliper.

FIG. 3 is a side view of the sizing caliper fitted against a distal endof a femur 50 in a retracted position.

FIG. 4 is a detailed side view of the sizing caliper in the retractedposition of FIG. 3.

FIG. 5 is a side view of the sizing caliper in a substantially openedposition.

FIG. 6 is an exploded view of the sizing caliper.

FIG. 7 is a detailed side view of the cross linkage configuration of thesizing caliper in retracted position.

FIG. 8 is a detailed side view of the cross linkage configuration of thesizing caliper in a substantially opened position.

FIG. 9 is a side view of a femoral prosthesis.

FIG. 10 shows a bottom side view of the sizing caliper showing anotheralternate embodiment of the coupling configuration between the baseportion and the caliper body.

FIG. 11 is a side view of another embodiment of the sizing caliper.

FIG. 12 is an exploded view of the sizing caliper according to anotherembodiment.

The features shown in the above referenced drawings are illustratedschematically and are not intended to be drawn to scale nor are theyintended to be shown in precise positional relationship. Like referencenumbers indicate like elements.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. In the description, relativeterms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,”“below,” “up,” “down,” “top” and “bottom” as well as derivative thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should beconstrued to refer to the orientation as then described or as shown inthe drawing under discussion. These relative terms are for convenienceof description and do not require that the apparatus be constructed oroperated in a particular orientation. Terms concerning attachments, suchas “coupled”, “connected” and “interconnected,” refer to a relationshipwherein structures are secured or attached to one another eitherdirectly or indirectly through intervening structures, as well as bothmovable or rigid attachments or relationships, unless expresslydescribed otherwise.

Referring to FIGS. 1, 2 and 3, a femoral sizing caliper 1 according toan embodiment is shown. The femoral sizing caliper 1 facilitates theselection of a femoral component of a knee prosthesis to be attached toa resected distal end 52 of a femur 50. In this embodiment, the sizingcaliper 1 is a linkage assembly comprised of a caliper body 2, a stylus3, a stylus tower 4, a linkage pin 5, drill guide body portions 6 a, 6b. The caliper body 2 can further comprise a base portion 7 that isconfigured to be positioned adjacent to one or more of the posteriorfemoral condyle surfaces 58 of the femur 50 for referencing the caliperbody to one or more of the posterior femoral condyles. The base portion7 comprises paddles 77 for contacting one or more of the posteriorfemoral condyle surfaces 58 that establish the orientation and positionof drill guide holes 62 a, 62 b.

FIG. 3 shows a side view of the femoral sizing caliper 1 engaging theresected distal end 52 of the femur 50. To determine the proper size forthe femoral component of a knee prosthesis, the caliper body 2 is placedflush against the resected distal end 52 of the femur 50 and the caliper1 is adjusted so that the paddles 77 of the base portion 7 rest againstone or more of the posterior condyle surfaces 58 and the tip of thestylus 3 rests against the most prominent aspect 57 of the anteriorcortex just proximal to the anterior condyles. The position of the tipof the stylus 3 shown in this embodiment can be adjusted by turning thestylus 3 about the pivot axle 44 of the stylus tower 4 using the handle33. Another embodiment of stylus 3 may permit proximal translation ofthe stylus tip to account for the varying proximal heights of theanterior flange of the different sizes of distal femoral implants. Thisadjustment allows the sizing caliper 1 to be properly fitted to thefemur 50 according to the anterior/posterior (A/P) dimension 8 of thefemur 50 for purpose of determining the optimal size for the femoralknee component. Once the sizing caliper 1 is properly adjusted to fitthe femur 50, the graduated size indicator markings 25 provided on thecaliper body 2 indicates the optimal size of the femoral component ofthe knee prosthesis for the femur 50.

As shown in FIGS. 1, 2 and the exploded view of the caliper shown inFIG. 6, the caliper body 2 is provided with openings 22 a, 22 b on bothsides to provide access to the drill guide holes 62 a, 62 b provided inthe drill guide bodies 6 a, 6 b, respectively. (See FIGS. 1 and 2). Withthe sizing caliper 1 properly fitted to the femur 50, referencing holes55 are drilled into the resected distal end 52 of the femur using thedrill guide holes 62 a, 62 b.

After the referencing holes 55 are drilled, the caliper 1 is removedfrom the femur 50 and the referencing holes 55 in the resected distalend 52 of the femur 50 are used to position a resection guide (notshown) which is used to make further resections to the distal end of thefemur 50. The resections form surfaces that correspond to internalplanar surfaces 75 on the interior surface of a femoral component 70 ofa knee prosthesis. FIG. 9 is a side view of an example of the femoralcomponent 70 showing the internal planar surfaces 75. The femoralcomponent 70 is provided with two medial/lateral (M/L) stabilizer pegs72 that are inserted into the referencing holes 55 in the femur whenbeing implanted in the patient's knee. In the conventionalanterior-referencing or posterior-referencing femoral component sizingschemes, the location of the drill guide holes provided on the sizingcalipers remain constant with respect to the corresponding referencingsurfaces (anterior cortex or the posterior condyles). For thisdisclosure, the femoral component 70 is an example of a prostheticcomponent designed with a midline-referencing sizing scheme. Themidline-referencing sizing scheme for the femoral component 70 requiresthat the stabilizer pegs 72 are always positioned in the middle of theA/P dimension 8 so that the distance between the center of thestabilizer pegs 72 and the posterior condyle surface 11 of the femoralcomponent 70 is (A/P)/2 dimension 9. The sizing caliper of the presentdisclosure allows sizing of the distal end of the femur 50 and locatingof the referencing holes 55 for receiving the stabilizer pegs 72 for themidline-referenced femoral component 70.

Unlike the sizing calipers used in the conventional sizing schemes, thesizing caliper 1 of the present disclosure allows midline-referencingsizing scheme for the femoral component 70 of the knee prosthesis. Toaccommodate the midline-referencing sizing scheme for the femoralcomponent 70, the location of the drill guide holes 62 a and 62 b in thesizing caliper 1 with respect to the A/P dimension 8 is always at themidpoint of the A/P dimension 8.

To achieve the midline-referencing, the sizing caliper 1 is configuredsuch that when the caliper body 2 and the stylus tower 4 are linearlydisplaced in opposite directions (drawn apart or brought together)relative to the drill guide body portions 6 a, 6 b in order to increaseor decrease the A/P dimension 8, the caliper body 2 and the stylus tower4 are linearly displaced at an equal rate relative to the drill guidebody portions 6 a, 6 b so that the drill guide body portions 6 a, 6 bmaintain the same relative position relative to the caliper body 2 andthe stylus tower 4. Therefore, the drill guide holes 62 a, 62 b that areprovided on the drill guide body 6 will maintain same position relativeto the paddles 77 provided on the caliper body 2 and the stylus 3 on thetower 4.

As shown in FIGS. 4 and 5, regardless of whether the sizing caliper 1 isin a substantially closed position as shown in FIG. 4 or in asubstantially open position as shown in FIG. 5, the location of thedrill guide holes 62 a, 62 b are always along the midpoint of the A/Pdimension 8 of the femur 50. Referring to FIGS. 7 and 8, this aspect ofthe sizing caliper 1 is accomplished by a cross linkage 13 formed by thecaliper body 2, the drill guide body portions 6 a, 6 b, the stylus tower4, and a linkage pin 5. FIG. 7 shows the configuration of the crosslinkage 13 corresponding to the substantially closed sizing caliper 1shown in FIG. 4. FIG. 8 shows the configuration of the cross linkage 13corresponding to the substantially open sizing caliper 1 shown in FIG.5.

The exploded view of FIG. 6 shows the structure of the individualcomponents of the sizing caliper 1. The cross linkage 13 is formed bythe caliper body 2, the drill guide body portions 6 a, 6 b and thestylus tower 4 assembled with one another and slidably linked to eachother by the linkage pin 5. The linkage pin 5 laterally extends throughthe caliper body 2, the drill guide body portions 6 a, 6 b and thestylus tower 4. The caliper body 2 has a middle portion 21 that ispositioned between the two drill guide body portions 6 a, 6 b. In themiddle portion 21 is provided a first cross linking slot 26 that isoriented at an angle α (see FIG. 7) relative to the longitudinal axis Lof the caliper 1 where the longitudinal axis L defines the directions ofthe sliding motions of the caliper body 2 and the stylus tower 4. Inthis embodiment, the stylus tower 4 is configured with two extendingportions 4 a, 4 b that extends into the caliper body 2 on either side ofthe middle portion 21. Each of the two extending portions 4 a, 4 b isprovided with a second cross linking slot 46 a, 46 b that are orientedat an angle β (see FIG. 7) relative to the longitudinal axis L. Theangles α and β of the cross linking slots 26 and 46 a, 46 b,respectively, are equal which allows the caliper body 2 and the stylustower 4 to be linearly displaced at an equal rate relative to the drillguide body portions 6 a, 6 b.

Although two second cross linking slots are provided on the stylus tower4 in this embodiment for symmetry, one of ordinary skill in the artwould understand that functionally, only one of the second set of crosslinking slots 46 a, 46 b is necessary. For example, one of the twoextending portions 4 a, 4 b can be provided with one cross linking slot46 and a large clearance hole can be provided on the other of the twoextending portions 4 a, 4 b. The clearance hole would be sufficientlylarge to not interfere with the sliding motion between the linkage pin 5and the second cross linking slot 46.

Each of the drill guide body portions 6 a, 6 b is provided with a thirdcross linking slot 66 a, 66 b, respectively, that is oriented transverseto the longitudinal axis L. The components, caliper body 2, drill guidebody portions 6 a, 6 b and the stylus tower 4 are assembled togetheraligning the cross linking slots 26, 46 a, 46 b, 66 a and 66 b in anoverlapping manner as shown in the side views FIGS. 7 and 8. The linkagepin 5 laterally extends through the cross linking slots 26, 46 a, 46 band 66 a, 66 b.

FIG. 7 shows the configuration of the sizing caliper 1 in a closed orretracted position. Starting with the configuration of the cross linkage13 shown in FIG. 7, as the caliper body 2 and the stylus tower 4 aredrawn apart from one another, the first and second cross linking slots26 and 46 a, 46 b corresponding to the caliper body 2 and the stylustower 4, respectively, move in the directions shown by arrows A and Brelative to the drill guide body portions 6 a, 6 b. As the caliper body2 and the stylus tower 4 are drawn apart in their respective directionsA and B, the angled orientation of the cross linking slots 26 and 46 a,46 b cause the linkage pin 5 to slide laterally within the slot 66 inthe direction perpendicular to the directions A and B represented by thearrow C. Thus, the linkage pin 5 does not move along the directions A orB and reflects the stationary position of the drill guide body portions6 a, 6 b in the longitudinal directions A and B relative to the stylustower 4 and the caliper body 2.

FIG. 8 shows the configuration of the cross linkage 13 after the caliperbody 2 and the stylus tower 4 are drawn apart where the sizing caliper 1is in a substantially open configuration. The letter D identifies theprevious positions of the cross linking slots 46 a, 46 b and the letterE identifies the previous position of the cross liking slot 26 where thecaliper 1 was in a substantially closed configuration shown in FIG. 7.Thus, in the configuration shown in FIG. 8, the stylus tower 4 has beenlinearly displaced by distance x and at the same time, the caliper body2 has been linearly displaced also by the same distance x. Thus, thedrill guide holes 62 a, 62 b provided in the drill guide body portions 6a, 6 b are always positioned at the midpoint 9 of the A/P dimension 8.

The stylus 3 that positions the A/P dimension 8 of the caliper 1 is in afixed position 14 in this embodiment but has the ability to rotate orcome in from either the left or right side of the assembly, depending onthe access of the surgical incision. However, given the variability ofthe implant, the stylus 3 design can be adjusted so that the measurementposition 14 can vary with the size of the implant.

In the illustrated embodiment of the sizing caliper 1, the base portion7 is a modular unit that removably couples to the caliper body 2. Thebase portion 7 and the caliper body 2 are configured to couple anddecouple using a sliding mechanism. As shown in FIG. 2, the base portion7 and the caliper body 2 are configured with tongue and groove likestructures 78, 28, respectively, to allow the base portion 7 to slideinto the caliper body 2. The modular construction allows provision ofreplaceable base portions each having different angles of externalrotation for the final femoral component.

Referring to FIG. 10, in another embodiment, the base portion 7 and thecaliper body 2 can be pivotally coupled to each other so that the angleof the base portion 7 can be adjusted without replacing the base portion7. In this embodiment, the base portion 7 is pivotally coupled to thecaliper body 2 by a spindle or a pin 72. Preferably, the spindle 72 isaligned with the longitudinal axis L of the caliper.

The configuration of the sizing caliper 1 is not limited to theparticular embodiment represented by the cross linkage 13 configurationformed by the cross linking slots 26, 46 a, 46 b, 66 a, 66 b and thelinkage pin 5 discussed above. For example, as shown in FIG. 11,according to another embodiment, a rack and pinion gear assembly can beimplemented to achieve the same function of allowing the caliper body 2and the stylus tower 4 to be linearly displaced in opposing directionsrelative to the drill guide body portions 6 a, 6 b at the same rate. Inthis embodiment, each of the caliper body 2 and the stylus tower 4 isconfigured with a teeth rack 27 and 47, respectively, and the linkagepin 5 is provided with a pinion gear 57. The linkage pin 5 is capturedbetween the caliper body 2 and the stylus tower 4 by the teeth racks.The pinion gear 57 links the linear motions of the caliper body 2 andthe stylus tower 4 and allows the caliper body 2 and the stylus tower 4to be linearly displaced in the opposite directions A and B at the samerate.

Even in embodiments, where cross linking slots and a linkage pin is usedto slidably link the components of the sizing caliper, the cross linkageconfiguration of the sizing caliper of the present disclosure is notlimited to the particular embodiment shown in FIG. 6. FIG. 12 shows anexploded view of a sizing caliper 100 according to an alternateembodiment. The sizing caliper 100 comprises a caliper body 102, drillguide body portions 106 a, 106 b, and a stylus tower 104. The stylustower 104, the drill guide body portions 106 a, 106 b and the stylustower 104 assemble together and slidingly coupled together by a linkagepin 105 similar to the sizing caliper 1. The stylus tower 104, however,is not bifurcated like the stylus tower 4 of the sizing caliper 1. Thestylus tower 104 has a single elongated body with a first cross linkingslot 146 (equivalent to the cross linking slots 46 a, 46 b) providedtherein. The caliper body 102 does not have a middle portion where asecond cross linking slot is provided in the caliper body 2 of thecaliper embodiment 1. Instead, the caliper body 102 is provided with twosecond cross linking slots 126 a, 126 b (equivalent to the cross linkingslot 26) on its sidewalls. Two drill guide body portions 106 a, 106 bare provided where each of the drill guide body portions 106 a, 106 b isprovided with a third cross linking slot 166 a, 166 b (equivalent to thecross linking slots 66 a, 66 b) and a drill guide hole 162 a, 162 b. Thestylus tower 104 fits between the two drill guide body portions 106 a,106 b and those components fit within the caliper body 102. As in thesizing caliper 1, in the caliper 100, the stylus tower 104, the caliperbody 102 and the drill guide body portions 106 a, 106 b are assembledtogether, thus aligning the cross linking slots 126 a, 126 b, 146, 166 aand 166 b in an overlapping manner similar to the overlapping alignmentof the equivalent cross linking slots shown in FIGS. 7 and 8. Thelinkage pin 105 laterally extends through the cross linking slots 126 a,126 b, 146, 166 a and 166 b and slidably links the caliper body 102, thestylus tower 104, and the drill guide body portions 106 a, 106 b.

Unlike the conventional femoral sizing calipers that reference fromeither the anterior cortex or the posterior condyles, the sizing caliperof the present disclosure allows the surgeon to reference from themidline of the A/P dimension 8 and thus position the desired componentat the midline of the A/P dimension 8. This splits the differencebetween both previously defined methods and permits optimal positioningof the final prosthesis.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodimentsof the invention, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention. Thescope of the invention disclosed herein is to be limited only by thefollowing claims.

What is claimed is:
 1. A sizing caliper instrument for facilitating theselection of a femoral component of a knee prosthesis to be attached toa resected distal end of a femur, said femur having one or moreposterior femoral condyles, said distal femur including an anteriorcortex, a distal condyle, and a midline of an anterior-posteriordimension, comprising: a caliper body having a posterior paddle thereon,a drill guide body having a midline reference thereon, and a stylustower having an anterior stylus thereon, the caliper body, the drillguide body and the stylus tower slidably linked to each other wherebywhen the caliper body and the stylus tower are linearly displaced inreference to the drill guide body, the paddle and the stylus arelinearly displaced at an equal rate with respect to the midlinereference of the drill guide body.
 2. The sizing caliper instrument ofclaim 1, further comprising the drill guide body having two drill guideholes, the drill guide holes located at the midline reference of thedrill guide body.
 3. The sizing caliper instrument of claim 2, whereinwhen the stylus tower and the caliper body are linearly displaced inreference to the drill guide body, the drill guide holes remain locatedat the midline of said anterior-posterior dimension.
 4. The sizingcaliper instrument of claim 2, further comprising the drill guide bodyhaving two spaced apart drill guide body portions, and one of the drillholes is located on each of the drill guide body portions.
 5. The sizingcaliper instrument of claim 4, wherein when the stylus tower and thecaliper body are linearly displaced in reference to the drill guide bodyportions, the drill guide holes remain located at the midline of saidanterior-posterior dimension.
 6. The sizing caliper instrument of claim1, wherein the caliper body, the drill guide body and the stylus towerare slidably linked to each other by a linkage pin laterally extendingtherethrough.
 7. The sizing caliper instrument of claim 1, wherein thestylus is graduated and is slidably coupled to the stylus tower.
 8. Thesizing caliper instrument of claim 7, wherein a linkage pin extendsthrough the caliper body, the drill guide body portions, and the stylustower.
 9. The sizing caliper instrument of claim 4, wherein the caliperbody is provided with a first cross linking slot that is oriented at anangle alpha relative to a longitudinal axis L; the stylus tower providedwith at least one second cross linking slot that is oriented at an anglebeta relative to the longitudinal axis L; each drill guide body portionprovided with a third cross linking slot that is oriented transverse tothe longitudinal axis L; the caliper body, the two drill guide bodiesand the stylus tower are assembled to be slidably linked to each otherby aligning the first cross linking slot, the at least one second crosslinking slot and each third cross linking slot in an overlapping manner;the caliper further comprising a linkage pin laterally extending throughthe first cross linking slot, the at least one second cross linkingslot, and each third cross linking slot and slidably linking the caliperbody, the two drill guide bodies and the stylus to each other.
 10. Thesizing caliper instrument of claim 9, wherein the caliper body includesa base portion removably coupled to the caliper body.
 11. The sizingcaliper instrument of claim 10, wherein the posterior paddle includes apair of paddles on the base portion.
 12. The sizing caliper instrumentof claim 9, wherein the caliper body includes a base portion pivotallycoupled to the caliper body.
 13. The sizing caliper instrument of claim12, wherein the posterior paddle includes a pair of paddles on the baseportion.
 14. The sizing caliper instrument of claim 4, wherein thecaliper body, the two drill guide body portions, and the stylus towerare slidably linked to each other by a rack and pinion configuration.15. The sizing caliper instrument of claim 14, wherein the caliper bodyand the stylus tower are each provided with a teeth rack and theslidable link is provided by a pinion gear that engages the teeth racks.